1. Field of the Invention
The invention relates to a hybrid electrical switching device, comprising an electromechanical relay with a by means of an electrical coil operational mechanical switching element and a main semiconductor switching element with a control input and a parallel with the mechanical switching element connected current conduction path.
2. Description of Related Art
A device of this kind is known from U.S. Pat. No. 5,790,354.
The term hybrid electrical switching device is derived from the combination of a mechanical switching element and a semiconductor switching element.
The known switching device operates such that when the mechanical relay is operated, the parallel to the mechanical switching element connected semiconductor switching element is simultaneously brought in its conducting state. Since the semiconductor switching element is faster in its conducting state than the mechanical switching element, the electric load being switched by the switching device is switched on faster as compared to a similar electromechanical relay without a parallel-connected semiconductor switching element. In other words, the semiconductor switching element eliminates the influence of the pull-in or switch-on delay of the electromechanical relay.
By placing the semiconductor switching element in its conducting state not only upon switching on, but also upon switching off of the mechanical relay, damaging of the contacts of the mechanical switching element caused by arcing and sparking is reduced, which furthermore effects a significant reduction of the power dissipation of the switching device as a whole.
However, the known hybrid electrical switching device has a number of inherent drawbacks.
In the case of an insufficiently conducting mechanical switching element, for example due to ageing and/or fouling of the switching contacts thereof, but also in the case of failure of the mechanical switching element when a load is switched on, a part or even the entire load current will be able to flow through the semiconductor switching element for a relatively long period of time. In order to prevent the semiconductor switching element from being damaged, it must be dimensioned sufficiently “heavy”. That is, at least equal to the maximum load current of the electric load to be switched with the switching device.
The known switching device further requires a fairly extensive control circuit, in which the semiconductor switching element furthermore needs to be of an optically controlled type. Since the coil of the mechanical relay is connected in series with a part of the control circuit, this switching device is not naturally suitable for controlling electromechanical relays with coils suitable for usual voltages as they are used in electrical installations for households and the like, i.e. with a typical voltage of 230 V.